Method and device for anaerobic fermentation of solid organic waste substances

ABSTRACT

For the anaerobic fermentation of solid organic waste substances, solid waste is mixed with liquid material, particularly anaerobic slurry, and the temperature of the mixture is brought to a value between 25° and 70° C., preferably between 30° and 40° C. (mesophilic) and between 55° and 65° C. (thermophilic). By means of spontaneous rising of lightweight material and by means of flotation, a layer (10) of solid material floating on a methane-generating zone is formed from said mixture in a reactor. Said floating layer moves from a supply end (7) or a mixing section (2) to a discharge end (14) of the reactor, hydrolysis and acidification of solidorganic material taking place in the floating layer. The fermented floating layer is discharged via the discharge (14) end independent of the residence time of fluid and slurry in the methane-generating zone under the floating layer.

In the first instance, the invention relates to a method for theanaerobic fermentation of solid organic substances in a reactor tank inwhich there is a mixture of the solid organic substances and ananaerobic fluid, in which a layer of material floating on amethanegenerating fluid is moved from a supply end to a discharge end ofsaid reactor tank and a methane-forming reaction is induced in themethane-generating fluid under the floating layer, and in which fluid issprayed in and/or on the floating layer.

Such a method is described in U.S. Pat. No. 4334997.

Solid waste substances may consist, inter alia, of vegetable. fruit andgarden waste, household waste and organic industrial waste.

Purification of sewage and the processing of manure has, for decades,employed fermentation processes. Fermentation leads to the production ofbiogas and to stabilization of waste or slurry. Increasingly, however,fermentation processes are also being used to process waste from theagro-industry and household waste (such as vegetable, fruit and gardenwaste).

When use is made of a completely mixed reactor or a plug-flow reactor,the residence times of the waste to be fermented and of the biomass(methane-generating sludge) are identical to each other. However, thegrowth rate of methane-generating bacteria is relatively low, whichresults in the residence time of the biomass and thus of the material tobe fermented having to be relatively long (20 to 30 days). This resultsin relatively long reactor tanks. Although systems are known in which aseparation is brought about between the residence times of fermentingmaterial and methane-generating biomass, they usually make use of aplurality of reactors with complicated separation systems between them.This also leads to high production and operating costs.

In the method according to the abovementioned US patent specification,the floating layer is moved and discharged independent of themethane-generating fluid zone. In other words, the residence times ofthe floating layer and the methane-generating slurry do not have to beidentical to each other. The fluid sprayed on the floating layerconsists of deoxygenated water which fulfils only a transport functionfor the floating layer. The fluid will have to be vigorously squirtedonto and in the floating layer, which will also have a mixing effectand, as a result of this, solid portions of the floating layer aresquirted into the methane-generating fluid, to the detriment of thethickness of the floating layer. In each case, no attempt is made toferment the components of the floating layer. The floating layer isregarded only as an inconvenience and is therefore kept as thin aspossible. The floating layer is discharged from the reactor tank asquickly as possible.

The object of the invention is to generate a controlled fermentationreaction in the floating layer.

To this end, the method mentioned in the preamble is characterized inthat the fluid sprayed in and/or on the floating layer is extracted fromthe methane-generating zone under the floating layer in order to inducefermentation in the floating layer and also, by means of percolation, toremove acid fermentation products from the floating layer and to drivethem to the methane-generating zone under the floating layer, and inthat the floating layer is moved in such a controlled manner that thesaid fermentation reaction can take place in the floating layer.

Solid waste and anaerobic slurry could be mixed outside the reactor, butit is preferable for this mixing to be carried out in a mixing sectionof the actual reactor.

During mixing, heavy material which has sunk has to be removedperiodically.

The anaerobic slurry will have to be brought to the desired temperaturein order to achieve adequate fermentation performance levels. In thecase of the mesophilic bacteria, this means that the temperature has tobe brought to between approximately 30° and approximately 40° C., whilstin the case of thermophilic bacteria, a temperature of betweenapproximately 55 and approximately 65° C. is favourable.

According to the invention, by means of spontaneous floatation of thelight solid material and sinking of the biomass, a separation is broughtabout between said two materials, and both materials are fermentedadequately independently and in their own time. It is essential thatfluid from the methane-generating zone is sprayed in and/or on thefloating layer, by means of which fermentation in the floating layer isachieved.

The biogas formed in the methane-generating zone flows upwards andbubbles through the floating layer, as a result of which, on the onehand, the floatation is enhanced and, on the other hand, mixing andbreaking-open of the floating layer take place. Mixing can beintensified if the fluid withdrawn from the methane-generating zone isalso used to improve the intake of solid material and mixing.

The 1- to 2-meter-thick floating layer will be broken up to a certaindegree and the contents of the reactor will be better mixed if thebiogas formed is at least partially recirculated by being injected, atdifferent locations, into the lower portion of the reactor.

The residence time of the fermenting material in the floating layer inthe reactor is approximately 5 days. During this time, the floatinglayer is moved towards the discharge end of the reactor using mechanicalmeans.

At the discharge end of the reactor, the floating layer is pushed undera baffle forming part of a water seal or is removed via anothermechanism. The baffle can be adjusted in order to regulate the thicknessof the floating layer.

The invention also relates to a reactor for implementing the abovemethod, comprising a reactor tank with a supply end for a mixture ofsolid waste and anaerobic fluid and a discharge end for a layer floatingon the methane-generating fluid, means for moving the floating layerfrom the supply end to the discharge end, and means for spraying fluidin and/or the floating layer and means for discharging the floatinglayer out of the reactor, via a water seal, at the discharge endindependent of the fluid and slurry located under the floating layer.

Such a reactor is also known from said U.S. Pat. No. 4334997.

In order to be able to implement the method according to the invention,the means for spraying fluid in and/or on the floating layer areconnected to lines which can extract fluid from the methanegeneratingzone under the floating layer.

In this case, means may be present for discharging the biogas formed inthe reactor which are installed at different locations at the bottom ofthe reactor.

The means for moving the floating layer may consist of a hydraulicallymovable blade which can hinge in the forward direction of the floatinglayer, and can execute a downward translational movement in the floatinglayer during said hinging movement, and. when it has reached anapproximately vertical position can move upwards to a position outsidethe floating layer and, finally, can hinge back to the startingposition. A blade could also be moved mechanically to follow aparallelogram-shaped path: in sequence, obliquely forwards, in thedirection of the discharge end of the reactor, obliquely back upwardsand, finally, in the direction of the supply end of the reactor.

The invention will now be described in greater detail on the basis ofthe figure which gives a diagrammatic illustration of the reactoraccording to the invention.

The figure shows a reactor tank 1 which is closed at the top, consistingof a mixing part 2 and a fermentation part 3.

A supply conveyor 4 for solid organic waste, such as vegetable, fruitand garden waste, opens out, via its discharge end, above a rotatingdrum sieve 5 which separates the very coarse material, such as branches,car tyres and concrete blocks from the waste for fermentation formed bythe material which passes through the sieve. The very coarse waste fallsonto a storage area 6 and the material passing through the sieve fallsvia an inlet 7, designed like a water seal, into the mixing part 2. insaid part, stone, glass, ceramic material, metal and coarse sand sink tothe bottom and the newly arrived components to be fermented andmethane-generating biomass in the reactor are mixed. Then, in the mixingpart, the mixture is heated to a temperature of between 25° and 70° C.,preferably between 30° and 40° C. (mesophilic) or between 55 and 65° C.(thermophilic) by means of heating/mixing units 8. The latter onesconsist of double-walled vertical nozzles in whose cavity hot waterflows. From time to time, the material which has sunk and is lying onthe bottom of the mixing part is removed by means of a grab-crane 9.

In the fermentation portion 3, the actual biological conversions takeplace, as a result of which, owing to the rising up of the fibrousmaterial and flotation, caused by rising bubbles of biogas, a floatinglayer 10 is formed with a thickness of 1 to 2 meters. Said layer ispushed by means of hydraulically driven blades 11 in the direction ofthe discharge end 14 of the reactor, which discharge end is designed asa water seal. Before said discharge end there is a baffle 13 and aplunger 12 for pushing the fermented material of the floating layer 10under the baffle 13 through towards the water seal 14, which allows thematerial to fall onto a discharge area. The blades 11 can hinge in theforward direction of the floating layer and, simultaneously, describe adownward translational movement. When said blades have reached avertical position, they move upwards and hinge towards the startingposition.

During the approximately 5-day-long transportation of the material inthe floating layer, from the intake 7 to the outlet 12, organic materialis hydrolysed and acidified, while methane is also formed in thefloating layer. At a pH of between 6 and 7, solid substances, such asstarch and protein, are biologically converted into dissolved substancessuch as sugar, acetic acid and amino acids. The acidification productsmust not accumulate because, in the event of a highly acidified floatinglayer, the biological decomposition process will be halted. Discharge ofacidification products from the floating layer towards themethane-generating zone underneath takes place by percolation throughthe floating layer of buffer fluid which is drawn off by means of lines16 and sieves 17 from the methane-generating zone and is returned vialines 18 and spraying heads 19 in and/or on the floating layer. Thedrawn-off fluid also contributes to fermentation in the floating layer.

The dissolved acidification products are converted into biogas in themethane-generating biomass under the floating layer 10. Said biogasflows upwards and leaves the reactor via the line 2C after it hasbubbled through the floating layer, which gives rise to extra mixing andopening-up of the floating layer. As a result of this, there will be noformation of a crust, blind spots or dead spaces.

The biogas flows via the line 20 to a tank 21 in which foam is separatedoff. From there, the gas flows via a discharge line 22 towards, forexample, a generator. A branch line 23 of the line 22 carries a portionof the biogas, after the latter has passed through a compressor 24, to anumber of lines 27 opening out in the lowermost portion of the reactor.Said extra biogas intensifies the mixing in the methane-generating zoneand causes greater opening-up of the floating layer.

The fluid drawn off from the methane-generating zone via the line 16can, in addition to being used for said percolation through the floatinglayer, be used for improving the supply and mixing of the fermentedmaterial (see line 25) and for spraying the material which has beencollected by the crane 9 (see line 26).

The degree of acidity in the floating layer will be between 6 and 7 andthat in the methane-generating slurry between 7 and 8.

The most important advantage of the reactor described and the methoddescribed is that the residence times of the floating layer andmethane-generating zone may be different without complicated apparatus.The residence time of the material in the floating layer isapproximately 5 days and that in the methane-generating zone, dependingon the dry-matter content (which is usually less than 7%) 20 days, forexample. Furthermore, the fermentation in the floating layer itself isenhanced.

The fermented material discharged can, for example, be processed intocompost.

The slurry discharged from the methane-generating zone can be separatedout in a centrifuge, resulting in water on the one hand and a solidsubstance and methane bacteria on the other.

Various modifications and additions are possible within the scope of theinvention.

We claim:
 1. Method for the anaerobic fermentation of solid organicsubstances in a reactor tank (1) in which there is a mixture of thesolid organic substances and an anaerobic fluid, in which a layer ofmaterial (10) floating on an anaerobic methane-generating fluid is movedfrom a supply end (7) to a discharge end (14) of said reactor tank and amethane-forming reaction is induced in the anaerobic methane-generatingfluid under the floating layer (10), and in which fluid is sprayed inand/or on the floating layer (10), characterized in that the fluidsprayed in and/or on the floating layer is extracted from the anaerobicmethane-generating zone under the floating layer (10) in order to inducefermentation in the floating layer and also, by means of percolation, toremove acid fermentation products from the floating layer and to drivethem to the anaerobic methane-generating zone under the floating layer,and in that the floating layer (10) is moved in such a controlled mannerthat the said fermentation reaction can take place in the floatinglayer.
 2. Method according to claim 1, characterized in that mixing ofthe solid waste and anaerobic methane-generating fluid takes place in amixing section (2) of the reactor.
 3. Method according to claim 1,characterized in that, during mixing, heavy material which has sunk isperiodically removed.
 4. Method according to claim 1, characterized inthat fluid from the anaerobic methane-generating zone is also used toimprove the intake of solid material and mixing.
 5. Method accordingclaim 1, characterized in that said methane is at least partiallyrecirculated by being injected, at different locations, into the lowerportion of the reactor (1).
 6. Method according to claim 1,characterized in that the floating layer (10) is moved to the dischargeend (14) of the reactor using mechanical means (11).
 7. Reactor for theanaerobic fermentation of solid organic substances, comprising a reactortank (1) with a supply end (7) for a mixture of solid waste andanaerobic methane generating fluid and a discharge end (14) for a layer(10) floating on the anaerobic methane-generating fluid, means (11) formoving the floating layer from the supply end (7) to the discharge end(14), and means (19) for spraying fluid in and/or on the floating layerand means (12, 13) for discharging the floating layer out of thereactor, via a water seal (14), at the discharge end independent of thefluid and slurry located under the floating layer, characterized in thatthe means (19) for spraying fluid in and/or on the floating layer areconnected to leads (18) which can withdraw the fluid from the anaerobicmethane-generating zone under the floating layer (10).
 8. Reactoraccording to claim 7, characterized by means (27) for discharging themethane formed in the reactor and to supply said methane at differentlocations in the bottom of the reactor.
 9. Reactor according to claim 7,characterized by mechanical means (11) for moving a floating layer (10)formed in the reactor (1) in a controlled manner to the discharge end(14) of the reactor.